Fluid strainers have been employed for many years in which accumulated solids or sedimentation from the fluid is collected on a screening media and thereafter removed from the screening media by means of a hollow backwash arm in which a portion of the strained fluid or a different fluid from an external source is caused to flow in a reverse direction through the screening media. See, for example, U.S. Pat. Nos. 3,256,995, 4,315,820 and 4,328,103. Typical applications of such automatic self-cleaning strainers include removing unwanted debris from water, waste water and other fluid straining applications throughout industrial, utility, nuclear, petrochemical, commercial and municipal operations. These strainers may also be used in closed or open fluid systems. For example, air conditioning systems used in cooling tall buildings may operate during the day as a closed system, but may operate at night as an open system while the cooling water is directed through a cooling tower and exposed to cooler air as it passes over a cooling grill. The water may become dirty from the air, birds, leaves, or the like when the air conditioning system is operating as an open system.
To ensure that the screening media remains unclogged, backwashing could be operated continuously. However, continuous backwashing is not economical because far too much backwash water would be utilized, thus reducing the efficiency of the overall system. In addition, the added energy to continuously backwash is wasteful. It has been recognized in the art that less than continuous backwashing is desirable.
The two known methods of operating the backwashing in an automatic self-cleaning strainer are a timed method and a differential pressure sensing method. In the timed method, the backwashing mechanism is operated periodically for a certain time duration. In prior art systems, this timed method has included only one duration and one cycle time, the most typical of which is backwashing for three minutes out of every ten minutes. In the differential pressure sensing method the pressure drop across the strainer is monitored. As the strainer becomes clogged, less and less fluid can flow therethrough and, thus, the pressure drop across the strainer increases. When the pressure drop across the strainer exceeds a predetermined upper limit, the backwashing operation is initiated and is continued until the pressure drop across the strainer is reduced to a predetermined lower limit. Recognizing that removed debris may remain adjacent the screening media, the prior art has taught that it is desirable to continue the backwashing operation for a certain time delay past the point in time that the pressure drop reaches the lower limit. Backwashing for this extra time delay prevents the screening media from becoming immediately blocked again by debris which has not been completely removed from the strainer. The prior art systems have included a fixed time delay period for this extra backwashing, typically sixty seconds.
Usually both the timed method and the differential pressure method of operating a backwash control system are utilized together.
A disadvantage to the prior art systems described above is that the duration and cycle time of the timed backwash operation, and the time delay of the differential pressure backwash operation, are fixed and may not be appropriate for the particular system operating conditions. Much backwash water and energy may be wasted if the conditions required a smaller amount of water to clean the screening media.
Accordingly, it is an object of the present invention to provide an automatic, self-cleaning straining system in which the backwash operation can be adjusted in accordance with the operating conditions of the system.
It is another object of the present invention to provide a backwashing control system in which a minimum amount of backwash water and energy consumption is used to clean the screening media.